1. Field of the Invention
This invention relates to a new system for detection of structural failures. More particularly, the invention relates to a new and improved system for detection of cracks in metal structures.
Specifically, the invention provides a new and highly efficient in-situ on-line structural failure detection system for use in detecting cracks in metal structures, which system comprises a thin film adhesively bonded to the metal structural surface to be monitored, said film containing a plurality of thin continuous metal strips which are adapted to be broken when a crack appears under the strips, said strips being substantially parallel but insulated from each other and the metal structural surface and arranged in a pattern such that there is frequent change in direction of the strips as in a zig-zag or rectangular pattern, each of the said strips constituting a separate circuit joined to an electric power source and a sensing and recording means capable of detecting and recording any circuit failure as may be caused by disruption of the metal strip, said disruption being caused by the formation of a crack under the said strip.
Also provided is a method for preparing the above-noted detection systems and a method for their operation in actually detecting structural failures as evidenced by the appearance of cracks in the metal structure being monitored.
2. Prior Art
It is well known that under certain conditions critical parts in an aircraft structure are subject to failure by fatigue. Even though considerable attention and care is given in designing an aircraft structure from the point of structural integrity, the probability of failure cannot be entirely eliminated. This finite, however, small probability of failure is of great concern to industry.
During the past two decades a great deal of research work has been done to model fatigue crack growth behavior with the ultimate aim of predicting lifetime of a structural member subject to fatigue failure. Success rate in such predictions has been less than adequate or poor for a variety of reasons. One of the major difficulties in predicting lifetime of a structural member lies in the fact that in actual service the loading is irregular and not the typical periodic one used in most laboratory studies. Several research and development laboratories have attempted to incorporate spectrum loading on a laboratory scale through very elaborate systems in order to simulate the conditions experienced by an aircraft during takeoff, actual flight, and landing situations. Unfortunately, a complete load-time history for an aircraft cannot be simulated in the laboratory and the effects of an impulse overload can neither be predicted nor be realistically modelled. This is evidenced by the fact that an accurate lifetime prediction cannot be made with reasonable certainty using any existing or forseeable theory on fatigue failures. An accurate lifetime preduction of a member subject to failure by fatigue is further complicated by the fact that several aluminum and titanium alloys either strain soften or strain harden depending upon existing conditions. In a recent paper, "Fatigue Crack Growth under Spectrum Loads", ASTM STP 595, 23 (1975), Schijue discusses the present understanding of fatigue crack growth under spectrum loading and has assessed the accuracy of crack growth predictions based on existing theories. It is important to emphasize that the load-type recording program does not give a direct measure of damage introduced during operations.
Due to this unreliability of failure predictions made by laboratory modelling, it has been long recognized that periodic examination of aircraft structures for defect detection is extremely vital. Several defect or crack detection systems which can be used for this purpose when the aircraft is in operation have been proposed. However, these systems often have certain limitations. These systems consist of a variety of probes and detection systems. The surface of a structural member must be scanned with the probes to obtain a signal and this signal in turn must be processed to detect a possible crack. During routine maintenance, it is extremely cumbersome and difficult and in some cases entirely impossible to examine all the critical parts of an aircraft; for example, an inner inaccessible structural component or a bolt joining an engine to a pylon. It is in such areas that if a crack should develop and grow to a critical size and go undetected, catastrophic failure could occur. It is entirely possible that many of the recent crashes of commercial aircraft may have resulted from fatigue failures of structural members that could not be properly examined for defects during routine maintenance.
The problem is not only one of recognizing the exist of a fatigue crack but also the extent of the failure, e.g. the length of the crack. In this regard, use of a crack detection gage has received considerable attention. However, this technique requires attaching a precracked gage on a structural member to a variety of loading conditions. Crack lengths in the member and the gage are experimentally determined as a function of time and are also compared with results obtained by theoretical analysis. Thus, this technique is merely an indirect method for determining crack lengths and of little value for commercial operations.
Various methods, such as disclosed in U.S. Pat. No. 4,026,660, U.S. Pat. No. 3,509,942, U.S. Pat. No. 4,106,332 and U.S. Pat. No. 3,831,171, have been proposed to solve the above-noted problems as to the detection of structural failure. However, none has been entirely satisfactory. These prior known methods, for example, do not provide an on-line process for detecting structural failures which provides an effective means for determining the initiation and propagation, or velocity and direction, of the failure, or methods for detecting failures in inaccessible areas, such as bolts, and the like.
It is an object of the invention, therefore, to provide a new system for detecting structural failures. It is a further object to provide a new and highly efficient in-situ on-line structural failure detecting system for use in detecting cracks in metal structures. It is a further object to provide a system for detecting fatigue failures as soon as they occur. It is a further object to provide a system for detecting fatigue failures which can be used during operation of the structure. It is a further object to provide a system for detecting fatigue failures which has no adverse effect on the operation of the system or the surrounding materials. It is a further object to provide a system for detecting fatigue failures which avoids the use of large circuitry. It is a further object to provide a system for detecting fatigue failures which also indicates the direction of the fatigue crack as well as the approximate length thereof. It is a further object to provide a system for detecting fatigue failures that is operative even in inaccessible areas. These and other objects of the invention will be apparent from the following detailed description thereof.